PREDICTING CHANNEL STABILITY AND ANALYZING CHANGES IN RIPARIAN COVER AND WOODY DEBRIS IN THE CHICKLEY RIVER WATERSHED, HAWLEY, MASSACHUSETTS, USA

River restoration actions are rarely analyzed over long time periods or compared with no action alternatives. Here we examine three common river restoration goals: increasing channel stability, riparian cover, and woody debris. Channel stability is the ability of a stream to transport water and sediment while maintaining channel geometry without severe erosion or deposition. Stable channels tend to have a Shields parameter (θ) of ≈ 0.05, and the slope, grain size, and depth may adjust to meet this value. However, θ has seldom been used to predict channel stability, based on the corollary hypothesis that if θ ≠ 0.05, then the channel is unstable. The Chickley River (Hawley, MA, USA) provides a natural laboratory to test these predictions due to a series of condition-altering events, including channelization (~1950), debris removal (~1950 to 2010), extreme flooding (Tropical Storm Irene, 2011), channelization and debris removal (2011), and subsequent restoration (2012). We use 1-dimensional HEC-RAS modeling to calculate θ at 15 repeatedly surveyed cross sections along an 8-km reach at three different timeframes: channelized pre-restoration (2011), post-restoration (2012), and four years post-restoration (2016). Results show θ = 0.08 ± 0.03 in channelized conditions (2011), and field observations of knickpoints and bed erosion indicate unstable conditions. Post restoration (2012), θ = 0.06 ± 0.02, and field data indicate stable channels from 2012 to 2015. Control reaches show θ = 0.05 ± 0.03 and little change from 2012 to 2015, suggesting stability in reaches flooded but not recently channelized or restored. Repeat aerial imagery analyses (2010, 2011, 2014) show the mean canopy opening (the inverse of riparian cover) increased from 5.8 m pre-flood to 12.3 m post-flood to 17.5 m post-channelization, but did not decrease due to restoration. Large woody debris increased from ~ 30 pieces per km pre-flood to ~330 pieces per km post-flood, then decreased to 0 pieces per km post-channelization, then increased to ~ 30 pieces per km post-restoration. These results show that flooding improved channel stability and woody debris occurrence, but that the restoration had limited success. This suggests that extreme flooding can be an effective mechanism for improvement of biophysical conditions at local and watershed scales.